160 BUTTERCUP. 



c. Loose arrangement of the cortical parenchyma, with 

 intercellular spaces. 



d. Presence of chlorophyll in some cells. 



2. The pith region, sometimes hollow, occupying the central 

 axis of the stem. 



a. Size and form of the cells. 



b. Intercellular spaces. 



f. Relative extent of pith area. 



3. The vascular bundle region lying between pith and cor- 

 tex. 



a. Distribution of the bundles. 



b. Parts of a bundle pair. 



i. Phloem, just within the cortex. Observe the ex- 

 tent of each phloem strand, and the way in which 

 the phloem cells gradually grade into the 



ii. Cambium layer, composed of small thin-walled rect- 

 angular cambium cells. These actively growing 

 cells produce new tissues from their outer and in- 

 ner faces throughout the growing season. 



iii. Xylem, composed of cells whose peculiar thicken- 

 ings can only be seen in longitudinal section. In 

 the central part of an entire section will be seen the 

 xylem ring, outside of which is the cambium ring 

 that is in turn surrounded by the phloem ring. 

 Examine sections of both young and old stems to 

 see the differences in the amounts of xylem and 

 phloem present and the way in which they are 

 crowded together. 



4. Diagram the regions of the entire cross-section and illus- 

 trate in detail the tissues composing the older stem. 



II. THE LEAF. 



If satisfactory results were obtained in a study of the leaf 

 of Trillium, the study may here be omitted. Otherwise use 

 the leaf outline given under that plant. 



